WindXpert Series Tools for Architects, Engineers and Construction Professionals
Having a storm shelter, or a safe room built into your residential or commercial building can help you protect yourself, your family or your business from damage, injury or death caused by the dangerous forces of extreme winds.
Extreme winds are typically caused by high wind weather events such as Tornadoes and Hurricanes.
A storm shelter or a safe room are terms that are used interchangeably to define a section of your building that would be capable to resist these extreme high wind loads during a Tornado or a Hurricane.
The wind associated with these weather events can expose a building's envelope to extreme pressures and forces that are well above what the building had been designed for. Such extreme pressures would most likely partially or entirely damage your building.
Having a storm shelter would allow occupants of a building to be better protected and remain safer during an extreme weather event.
The effects of wind during a high wind weather event can cause the building to:
The actions from wind can cause damage to the building that may result the structural system of the building to become unstable and unsafe. This instability would expose the occupants to be unprotected from the effects of the weather event.
In addition, property valuables and non-replaceable personal belongings would not be protected during an extreme storm.
What is a Storm Shelter?
A storm shelter is usually a space within a building, or an entirely separate building designed and constructed to provide protection against both wind forces and the impact of wind-borne debris during an extreme weather event.
Shelters vary in design, configuration, and size. They can be a part the main building enclosure or they can be a separate building. They can be constructed above or below grade level.
To assist in the understanding of the criteria used to design and construct these shelters, we will emphasize on shelters that are a part of the main building and are enclosed within the main building of the property.
Basic Criteria for Shelter Design
Understanding the hazards, the differences and general damage associated to a building from the wind effects of a Tornado and a Hurricane would assist you in selecting the appropriate criteria for designing and constructing a storm shelter.
Criteria used in the design and construction of a shelter are different than criteria used in typical building construction. The level of protection for a shelter is intended to be much greater that the protection provided by buildings that comply with the minimum requirements of the building codes. Traditional building codes do not provide design and construction criteria for life safety for shelters nor do they provide design criteria for tornadoes.
The first step in determining your level of protection for your Property would be to determine your level of risk from being impacted by a high wind event.
Assess your Level of Risk for Tornadoes?
Determining your level of risk according to your building’s property geographical location would provide a basis in your decision of why you may benefit by a shelter. FEMA uses the term of "Safe Rooms" to distinguish between residential and community shelters.
Click the photo below to understand the different level of damages that Tornadoes and Hurricanes can generate.
Design Publications for Storm Shelters
There are three design recommendation publications that identify shelter criteria that are used in the design and construction of storm shelters in the United States.
Within each of the publications, wind and wind impact resistance are the key criteria used in the design and construction of storm shelters.
1. Wind Resistance
Buildings are designed to resist wind pressures that are based and calculated on range of design wind speeds. These design wind speeds are determined based on the geographical location of the property and have been outlined in national standards such as the American Society of American Engineers (ASCE) 7 Standard.
These design wind speeds have been determined based on historical hurricane data in combination with the probability of future hurricanes occurring in these areas. The highest design wind speed used in conventional construction near coastal areas of the Atlantic and Gulf Coasts of the US range between 115 to 180 miles per hour based on 3-second gusts according to ASCE 7-10 Standard. The design speeds used on storm shelter construction for these areas range from 200 to 250 miles per hour based on 3-second gusts as recommended by the Federal Emergency Management Agency (FEMA) while the ICC 500 Standard has wind speeds that range between 160 to 225 miles per hour. The higher design wind speeds from FEMA provide near absolute protection.
2. Wind Impact Resistance
During high wind weather event, debris tends to be moved by the high winds. This debris often referred to as wind-borne debris or missiles cause many injuries and damage to buildings. Exterior wall openings such as windows and doors and exterior walls are typically vulnerable to these missiles. These elements are part of the building envelope system of the building. To protect these exterior building envelope components, additional protection is usually provided. For window components, impact glass or shutters are usually used as protection.
For conventional building construction, the current building codes require protection for these building envelope components in some geographical locations throughout the US. These geographical locations are called “ wind-borne debris regions”. The ASCE 7 standard references the requirements of protection in wind-borne debris regions. The ASCE 7 missile criteria were developed to minimize property damage and improve the performance of the buildings. The criteria were not developed to protect occupants. Storm shelter criteria for impact resistance used in designing shelters consider much greater wind-borne debris impact loading.